The pixels in a liquid crystal display typically consist of a matrix of thin-film transistors (TFTs) which are used to transfer a voltage to the liquid crystal capacitor comprising each pixel of the display. Gray scale imaging using liquid crystal displays typically involve dividing each pixel into a plurality of subunits. A desired gray level is obtained by activating an appropriate number of such subunits. For example, U.S. Pat. No. 4,840,460 discloses a liquid crystal display that is subdivided into a plurality of subpixels. Each subpixel includes an effective capacitor, with the liquid crystal material contained between the capacitor plates. A control capacitor is coupled is coupled in series with the effective capacitor. The capacitance of the control capacitors can be controlled, thereby activating the subpixels as a function of the applied voltage across the series capacitance. Gray scale imaging is achieved by activating an appropriate number of subpixels for each pixel. U.S. Pat. No. 5,576,858 teaches a similar structure of subpixels. These approaches result in a complex pixel structure, and thus increase the manufacturing difficulties in liquid crystal panel fabrication.
A property of liquid crystal material is that the transmissivity of the material to light is proportional to the voltage applied to the material. While a high voltage level will cause the liquid crystal material to become opaque, exposing the material to lower voltages results in the attenuation of light passing through the material. Thus, by storing an appropriate charge at each pixel region in a liquid crystal layer gray scale imaging can be obtained using a much simpler structure than prior art approaches. However, a faithful reproduction of an image requires accurate storage of charge at each pixel.
Liquid crystal panels are commonly used in computer display systems. The proliferation of laptop units creates a demand for energy efficient displays, owing to the fact that a laptop has a limited independent source of power.
What is needed, therefore, is circuitry which can transfer a video signal to a plurality of pixels without degrading the quality of the signal. It is desirable to provide circuitry which, for the most part, operates at low voltage levels typical of CMOS devices, but which can operate at the high voltage levels typically encountered with the display of video signals on a liquid crystal panel. It is further desirable that low voltage operation be maintained whenever possible and that high voltage operation is active only during the creation of the image on the liquid crystal panel, thus keeping to a minimum the power requirement of the liquid crystal display.